The shedding of rat rod outer segment (ROS) discs, and the process of phagocytosis, is a circadian phenomenon characterized morphologically by the engulfment of ROS discs by the pigment epithelium. This process occurs over a period of a few hours beginning two hours after the start of the light period. The synthesis of ROS disc membrane phospholipids was also found to follow a circadian rhythm, being elevated during the daylight hours and depressed during the night. This rhythm persisted after three days of constant darkness as well as under constant light conditions. Phosphatidyl inositol exhibited an additional increase in synthesis in response to light at any time of the day or night.